1. Field of the Invention
This invention generally relates to electrochemical cells and, more particularly, to a transition metal hexacyanometallate (TMHCM)-conductive polymer (CP) composite battery electrode, and associated fabrication processes.
2. Description of the Related Art
Modern rechargeable lithium battery technology has triggered the portable electronic devices revolution due to high power density, long cycle life, and overall performance reliability. The rechargeable lithium battery consists of a cathode (positive electrode) and anode (negative electrode), separated by a lithium ion (Li+)-permeable membrane. A solution and/or polymer rich in Li+ is employed in order to ensure that lithium ions can freely migrate back and forth between the positive and negative electrodes. Common positive electrode materials include transition metal oxides such as lithium cobalt oxide (LiCoO2), lithium manganese oxide (LiMn2O4), and lithium iron phosphate (LiFePO4), in addition to various derivatives. Within these materials, lithium ions can move within their interstitial space(s) both freely and reversibly. In general, metallic lithium, alloys, and carbonaceous materials can be utilized as the negative electrode. When the rechargeable lithium battery does not include any metallic electrode, it is commonly referred to as lithium-ion battery (LIB). During the discharge process in a LIB, lithium ions are extracted from the negative electrode and subsequently inserted into the positive electrode. At the same time, electrons pass through an external circuit from the negative electrode to the positive electrode to generate electric power. During the charge process, ions and electrons move along the reverse directions and are restored to their original locations.
Although LIBs have been employed successfully over a broad range of commercial applications, lithium demand strains natural reserves, and potential fluctuations in price have motivated the development of a low-cost, rechargeable battery technology as an alternative to LIB. In light of this, sodium-ion batteries (NIBs) have received increased attention due primarily to the fact that sodium has similar properties to lithium, but also boasts the benefits of both reduced cost and virtually unlimited availability. However, similar to LIBs, NIBs require appropriate sodium ion (Na+)-host materials. Indeed, significant effort has been devoted to direct duplication of Li+-host structures for application as Na+-host electrode materials for NIBs. For example, NaCoO2, NaMnO2, NaCrO2, and Na0.85Li0.17Ni0.21Mn0.64O2, with an analogous layered-structure to LiCoO2, have been developed for NIBs. Similarly, Co3O4 with Spinel structure, Na3V2(PO4)3 with NASICON structure, and NaFePO4 with Olivine structure have been employed in sodium batteries. In addition, sodium fluorophosphates, such as Na2PO4F, NaVPO4F, and Na1.5VOPO4F0.5 have also been integrated as the positive electrode for sodium batteries.
Overall, it is impractical to simply adopt conventional Li+-host materials and structures as Na+ or potassium ion (K+)-host compounds, since Na+ and K+ are much larger than Li+ and, consequently, severely distort the structures of Li+-host compounds during the intercalation process. In light of this, it is critical to develop new Na+/K+-host materials with large interstitial space(s) through which Na+/K+ can migrate, both easily and reversibly. In general, both Na+ and K+ have been shown to readily intercalate into metal hexacyanometallate (MHCF) compounds. Widmann et al. demonstrated that K+ reversibly inserts/deinserts into/from copper, nickel, and iron hexacyanoferrates/hexacyanocobaltates [Prussian blue analogues (PBAs) comprising KCuFe(CN)6, KNiFe(CN)6, and KFeFe(CN)6] in an aqueous solution].1 Wessells et al. synthesized copper (KCuFe(CN)6) and nickel hexacyanoferrates (KNiFe(CN)6) and studied the behaviors of Na+/K+ insertion in aqueous media using a three-electrode cell.2,3 Overall, these results showed that capacities of the materials could achieve ˜60 milliamp-hours per gram (mAh/g). Eftekhari et al. assembled an iron hexacyanoferrate (Prussian blue)/potassium battery using an organic electrolyte comprising 1M KBF4 in ethylene carbonate/ethylmethyl carbonate (3:7).4 Overall, the results showed that Prussian blue was a robust electrode material for a potassium-ion battery and demonstrated a reversible capacity of ˜75 mAh/g. Finally, Lu et al. investigated a series of PBAs in a sodium battery with an organic electrolyte and reported that KFe(II)Fe(III)(CN)6 demonstrated the highest capacity (˜95 mAh/g), while KMnFe(CN)6, KNiFe(CN)6, KCuFe(CN)6, and KCoFe(CN)6 exhibited capacities of 50˜70 mAh/g.5 
Prussian blue and its analogues belong to a class of mixed valence compounds called transition metal hexacyanometallates (TMHCMs). In general, TMHCMs are characterized by a formula corresponding to AxM1mM2n(CN)6, where M1m+ and M2n+ are transition metals with different formal oxidation numbers (m and n). Usually, the transition metal hexacyanometallates may sequester a variety of different ions (A=Co+, Na+, K+, NH4+, Co2+, Cu2+, for example), as well as various amounts of water (H2O) within the crystal structure.
FIG. 1 is a schematic diagram depicting the framework of AxM1M2(CN)6 (prior art). The crystal structure of transition metal hexacyanometallates exhibits an open framework and is analogous to that of the ABX3 perovskite, as shown. M1m+ and M2n+ transition metal ions are in ordered arrangement on the B sites. The large, tetrahedrally coordinated A sites can host both alkali and alkaline earth ions (Ax) in addition to species such as H2O. The number of alkali (or alkaline earth ions) in the large cages of this crystallographically porous framework may vary from x=0 to x=2 depending on the respective valence(s) of M1 and M2. The open framework structure of the transition metal hexacyanometallates facilitates both rapid and reversible intercalation processes for alkali and alkaline earth ions (Ax). In general, sodium or potassium ions (Na+ or K+) are located at the A-site, as is also the case for Mg or Ca ions (Mg2+ or Ca2+).
In addition to the demonstrated potential for TMHCFs within the context of battery applications as described above, a set of unique electrochemical behaviors qualify these materials as promising candidates for alternate technologies including sensors for non-electroactive cations, transducers for hydrogen peroxide, enzyme-based biosensors, electrochromic devices, ion exchange media, electrocatalysis, and photoelectrochemical/photocatalytic devices.6-8 
In general, conductive polymers (CPs) or intrinsically conducting polymers (ICPs) are organic polymeric materials that conduct electrical charge and may exhibit either metallic conductivity or be semiconducting. Although a large variety of conductive polymers have been investigated, only a limited number have been reduced to practical (commercial) application due to both robust performance and acceptable levels of processsability. In particular, poly(acetylene)s, poly(p-phenylene vinylene), poly(pyrrole)s, poly(aniline)s, poly(thiophene)s, poly(3,4-ethylenedioxythiophene), and poly(p-phenylene sulfide) have been extensively investigated, both in terms of synthesis and properties. Despite commercial challenges for the wide-scale integration of conductive polymers including cost, poor to modest solubility, compositional/material performance inconsistencies, and lack of compatibility with direct melt processing, numerous potential applications have been suggested including photovoltaics (PV), printed electronics, organic light-emitting diodes (OLEDs), actuators, electrochromics, batteries/supercapacitors, chemical/biological sensors, flexible displays, and electromagnetic shielding, among others.
In general, the structural prerequisite for organic polymers to function as intrinsically conductive materials is the existence of a conjugated network (alternating single and double bonds) or, alternatively, conjugated units linked with atoms (such as sulfur or nitrogen) that possess p-orbitals capable of providing continuous orbital overlap. Furthermore, electronic conductivity within polymer systems requires not only the presence of charge carriers but also the facile migration of charge carriers, the latter of which can be accomplished through uninterrupted orbital overlap along the polymer backbone.
In light of the fact that most organic polymers do not possess intrinsic charge carriers and consequently behave as insulators in their native, updoped states, strategic “doping” is often employed. Accordingly, charge carriers may be introduced by means of either partial oxidation (p-doping) or partial reduction (n-doping). Overall, organic polymers can be “doped” through a variety of methods including chemical, electrochemical, photochemical, etc. For example, treatment of trans-polyacetylene with iodine affords a “doped”, highly conductive form of the material. Non-redox doping represents an alternative strategy for increasing the conductivity of conjugated polymers. In this case, the employed doping does not involve an alteration in the number of electrons associated with the polymer backbone, but rather includes a rearrangement of energy levels. A classic example of this type of doping is the treatment of polyaniline (PANI, emeraldine base form) with protic acids such as hydrochloric (HCl) or para-toluenesulfonic acid (p-TSA). In the case of PANI (emeraldine base) treatment with HCl to form the emeraldine hydrochloride, a conductivity enhancement approaching ten orders of magnitude may be realized. Finally, highly conductive forms of polypyrrole (Ppy) can be realized by oxidative (p-doping) with various chemical agents such as ferric chloride, although electrochemical oxidation is also possible. Overall, a vast majority of conventional conjugated polymers can be appropriately p- or n-doped (or both) to afford intrinsically conductive species through a wide variety of methods, although only a few examples have been provided.
Previously, Noël et al. described the preparation of an iron(III) hexacyanoferrate and poly(3,4-ethylenedioxythiophene) (PEDOT) composite on a platinum electrode using a potentiostatic method.9 Ogura et al. reported an investigation of ion transport during redox switching of a Prussian blue (PB)-polyaniline (PANI) bilayer electrode by employing an electrochemical quartz crystal microbalance (EQCM) and in situ Fourier transform infrared (FTIR) reflection spectroscopy.10 Kulesza et al. provided a method for fabricating composite organic/inorganic (hybrid) films on electrode surfaces using electrodeposition with potential cycling through which alternate layers of PANI and metal hexacyanoferrate were realized.11 Lupu et al. described the fabrication and electrochemical behavior of bilayer films of iron(III) hexacyanoferrate and poly[4,4′-bis(butylsulphanyl)-2,2′-bithiophene] on a platinum electrode.12 Somani et al. reported the electronic transport properties of electrochemically deposited conducting polypyrrole (Ppy)/PB bilayer films through an investigation of current-voltage characteristics under dark and white light illumination.14 Lisowska-Oleksiak et al. provided the synthesis of a PEDOT and PB analogue composite material for which a PB network was formed inside the PEDOT matrix using multicyclic polarization of an electrode.14 Feng et al. reported the one-pot synthesis of Ppy/PB and Ppy/Ag composite microtubes using methyl orange as a reactive, self-degrading template.15 Furthermore, the mechanism of formation, structural characteristics, conductivity, and electrochemical properties of the microtubes were reported.
Somani et al. investigated the electrochromic response of Ppy/PB composites in different electrolytes.16 The composite films were prepared through deposition of PB films on top of conductive Ppy films, whereby both films were fabricated by electrochemical methods. DeLongchamp et al. demonstrated the fabrication of a multiply colored electrochromic electrode using a layer-by-layer (LBL) assembly technique by exploiting intrinsic electrostatic attraction between a polycationic polymer (PANI) and a dispersion of negatively charged PB nanoparticles.17 Duluard et al. reported the assembly of organic/inorganic (hybrid) electrochromic devices [transparent conducting oxide (TCO)/inorganic counter electrode/hydrophobic electrolytic membrane/polymeric working electrode/TCO] featuring working electrodes consisting of polymer films prepared by in situ polymerisation of 3,4-ethylene dioxythiophene (EDOT) with galvanostatically deposited PB films as counter electrodes.18 
Curulli et al. described the preparation of a composite comprising conductive nanostructures of electropolymerized 1,2-diaminobenzene and PB on platinum electrodes wherein PB functioned as the active component for hydrogen peroxide (H2O2) detection.19 Ernst et al. provided structured films comprising PB realized through assembly within ultra-thin layers of 4(pyrrole-1-yl)-benzoic acid and PEDOT using alternate immersions.20 The composite film functioned as a redox conducting template for permanent attachment of horseradish peroxidase (HRP). Lupu et al. reported the preparation of PEDOT-PB films by a two-step method involving electrogeneration of a PEDOT film in the presence of ferricyanide ions with subsequent cycling of the composite electrode in an aqueous solution of ferric ions.21 The composite film demonstrated a high electrocatalytic effect towards dopamine oxidation in the presence of ascorbic acid. Miao et al. provided a reverse emulsion synthesis of PB/Ppy nanoparticles and subsequent immobilization on cysteine-modified gold electrodes.22 Furthermore, cyclic voltammetry confirmed a high electrocatalytic activity for the PB-Ppy materials towards hydrogen peroxide.
With respect to a sodium battery, the transition metal hexacyanometallate materials in conductive polymer composites may be represented by the general expression: Na2M1M2(CN)6, NaM1M2(CN)6, NaKM1M2(CN)6, and M1M2(CN)6, where M1, M2=Ti, Cr, Mn, Fe, Co, Ni, Cu, Zn, etc., and the ratio of M1 and M2 can be an arbitrary number. Some examples include: Na2Fe2(CN)6, NaFe2(CN)6, NaKFe2(CN)6, and Fe2(CN)6. With respect to a potassium battery, the transition metal hexacyanometallate materials in the conductive polymer composites may be represented by the general expression: K2M1M2(CN)6, KM1M2(CN)6, NaKM1M2(CN)6, and M1M2(CN)6, where M1, M2=Ti, Cr, Mn, Fe, Co, Ni, Cu, Zn, etc., and the ratio of M1 and M2 can be an arbitrary number. Some examples include: K2Fe2(CN)6, KFe2(CN)6, and NaKFe2(CN)6.
With respect to a magnesium battery, the transition metal hexacyanometallate materials in the conductive polymer composites may be represented by the general expression: MgM1M2(CN)6, Mg0.5M1M2(CN)6, and M1M2(CN)6, where M1, M2=Ti, Cr, Mn, Fe, Co, Ni, Cu, Zn, etc., and the ratio of M1 and M2 can be an arbitrary number. Some examples include: MgFe2(CN)6, Mg0.5Fe2(CN)6, Fe2(CN)6. With respect to a calcium battery, the transition metal hexacyanometallate materials in the conductive polymer composites may be represented by the general expression: CaM1M2(CN)6, Ca0.5M1M2(CN)6, and M1M2(CN)6, where M1, M2=Ti, Cr, Mn, Fe, Co, Ni, Cu, Zn, etc., and the ratio of M1 and M2 can be an arbitrary number. Some examples include: CaFe2(CN)6, Ca0.5Fe2(CN)6, and Fe2(CN)6.    (1) A. Widmann, H. Kahlert, I. Petrovic-Prelevic, H. Wulff, J. V. Yakhmi, N. Bagkar, and F. Scholz, “Structure, Insertion Electrochemistry, and Magnetic Properties of a New Type of Substitutional Solid Solutions of Copper, Nickel and Iron Hexacyanoferrates/Hexacyanocobaltates”, Inorganic Chemistry 2002, 41, 5706-5715.    (2) C. D. Wessells, R. A. Huggins, and Y. Cui, “Copper Hexacyanoferrate Battery Electrodes with Long Cycle Life and High Power”, Nature Communications 2011, 2, Article number: 550.    (3) C. D. Wessells, S. V. Peddada, R. A. Huggins, and Y. Cui, “Nickel Hexacyanoferrate Nanoparticle Electrodes For Aqueous Sodium and Potassium Ion Batteries”, Nano Letters 2011, 11, 5421-5425.    (4) A. Eftekhari, “Potassium Secondary Cell Based on Prussian Blue Cathode”, Journal of Power Sources 2004, 126, 221-228.    (5) Y. Lu, L. Wang, J. Cheng, and J. B. Goodenough, “Prussian Blue: A New Framework for Sodium Batteries”, Chemical Communications 2012, 48, 6544-6546.    (6) A. A. Karyakin, “Prussian Blue and Its Analogues: Electrochemistry and Analytical Applications”, Electroanalysis 2001, 13, 813-819.    (7) P. R. Somani and S. Radhakrishnan, “Electrochromic Materials and Devices: Present and Future”, Materials Chemistry and Physics 2002, 77, 117-133.    (8) N. R. de Tacconi, K. Rajeshwar, and R. O. Lezna, “Metal Hexacyanoferrates: Electrosynthesis, in Situ Characterization, and Applications”, Chemistry of Materials 2003, 15, 3046-3062.    (9) V. Noël, H. Randriamahazaka and C. Chevrot, “Composite Films of Iron(III) Hexacyanoferrate and Poly(3,4-ethylenedioxythiophene): Electrosynthesis and Properties”, Journal of Electroanalytical Chemistry 2000, 489, 46-54.    (10) K. Ogura, K. Nakaoka, and M. Nakayama, “Studies on Ion Transport During Potential Cycling of a Prussian Blue (inner) I Polyaniline (outer) Bilayer Electrode by Quartz Crystal Microbalance and Fourier Transform Infrared Reflection Spectroscopy”, Journal of Electroanalytical Chemistry 2000, 486, 119-125.    (11) P. J. Kulesza, K. Miecznikowski, M. A. Malik, M. Galkowski, M. Chojak, K. Caban, and A. Wieckowski, “Electrochemical Preparation and Characterization of Hybrid Films Composed of Prussian Blue Type Metal Hexacyanoferrate and Conducting Polymer”, Electrochimica Acta 2001, 46, 4065-4073.    (12) S. Lupu, C. Mihailciuc, L. Pigani, R. Seeber, N. Totir, and C. Zanardi, “Electrochemical Preparation and Characterisation of Bilayer Films Composed by Prussian Blue and Conducting Polymer”, Electrochemistry Communications 2002, 4, 753-758.    (13) P. Somani and S. Radhakrishnan, “Charge Transport Processes in Conducting Polypyrrole/Prussian Blue Bilayers”, Materials Chemistry and Physics 2002, 76, 15-19.    (14) A. Lisowska-Oleksiak, A. P. Nowak, and V. Jasulaitiene, “Poly(3,4-ethylenedioxythiophene)-Prussian Blue Hybrid Material: Evidence of Direct Chemical Interaction between PB and pEDOT”, Electrochemistry Communications 2006, 8, 2006.    (15) X. Feng, Z. Sun, W. Hou, and J-J. Zhu, “Synthesis of Functional Polypyrrole/Prussian Blue and Polypyrrole/Ag Composite Microtubes by Using a Reactive Template”, Nanotechnology 2007, 18, 195603.    (16) P. Somani and S. Radhakrishnan, “Electrochromic Response in Polypyrrole Sensitized by Prussian Blue”, Chemical Physics Letters 1998, 292, 218-222.    (17) D. M. DeLongchamp and P. T. Hammond, “Multiple-Color Electrochromism from Layer-by-Layer-Assembled Polyaniline/Prussian Blue Nanocomposite Thin Films”, Chemistry of Materials 2004, 16, 4799-4805.    (18) S. Duluard, A. Celik-Cochet, I. Saadeddin, A. Labouret, G. Campet, G. Schottner, U. Posset, and M-H. Delville, “Electrochromic Devices Based on in situ Polymerised EDOT and Prussian Blue: Influence of Transparent Conducting Oxide and Electrolyte Composition-Towards Up-Scaling”, New Journal of Chemistry 2011, 35, 2314-2321.    (19) A. Curulli, F. Valentini, S. Orlanduci, M. L. Terranova, and G. Palleschi, “Pt Based Enzyme Electrode Probes Assembled with Prussian Blue and Conducting Polymer Nanostructures”, Biosensors and Bioelectronics 2004, 20, 1223-1232.    (20) A. Ernst, O. Makowski, B. Kowalewska, K. Miecznikowski, and P. J. Kulesza, “Hybrid Bioelectrocatalyst for Hydrogen Peroxide Reduction: Immobilization of Enzyme Within Organic-Inorganic Film of Structured Prussian Blue and PEDOT”, Bioelectrochemistry 2007, 71, 23-28.    (21) S. Lupu, P. C. Balaure, C. Lete, M. Marin, and N. Totir, “Voltammetric Determination of Dopamine at PEDOT-Prussian Blue Composite Modified Electrodes”, Revue Roumaine de Chimie 2008, 53, 931-939.    (22) Y. Miao and J. 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It would be advantageous if the performance of TMHCM materials in alkali and/or alkaline earth batteries could be improved through the integration of conductive polymers.